This problem is meant for weaker players, and is by no means a challenge for higher-level players. Oh well, I preposterously assumed it was not that hard.
During this well-known variation of San-san invasion, Black builds up a wall, said to be uncuttable. White tries to probe this claim, backed up from far away. Assume all ladders work for White.
Thanks to Andre Engels for variations
(;GM[1]FF[4]CA[UTF-8]AP[CGoban:2]ST[2]
RU[Japanese]SZ[19]KM[0.00]
PW[Blanc]PB[Noir]AW[pb][qb][qc][qd][pe][re][rf]AB[ob][nc][pc][pd][qe][qf][qg]TR[pe]C[White played the marked stone. Engage !]PL[B]
(;B[oe]CR[oe]
;W[pf]CR[pf]
(;B[og]CR[og]
;W[od]CR[od]C[It looks like a net, but is it a genuine one ? White insists anyway. Prove him he's wrong !]
;B[oc]CR[oc]
(;W[of]CR[of]C[CHOICE]
(;B[nf]CR[nf]
;W[ne]CR[ne]
(;B[pg]CR[pg]
;W[oe]CR[oe]
;B[me]CR[me]C[CHOICE RIGHT
Stones are caught - use navigate for variations.])
(;B[me]CR[me]
;W[ng]SQ[qe][qf][qg]CR[ng]C[CHOICE WRONG
White escapes and the marked stones are in trouble now.])
(;B[nd]CR[nd]
;W[ng]SQ[qe][qf][qg]CR[ng]C[CHOICE WRONG
White escapes and the marked stones are in trouble now.]))
(;B[pg]CR[pg]
;W[nf]CR[nf]C[CHOICE WRONG
White escapes, and Black's influence is no more])
(;B[nd]CR[nd]
;W[nf]CR[nf]C[WRONG
White escapes, and Black's influence is no more]))
(;W[nd]CR[nd]
;B[md]CR[md]
;W[ne]CR[ne]
(;B[me]CR[me]
;W[of]CR[of]
;B[nf]CR[nf]C[RIGHT
White is caught here too
])
(;B[nf]CR[nf]
;W[me]CR[me]
;B[le]CR[le]
;W[mf]CR[mf]
;B[mg]CR[mg]
;W[lf]CR[lf]C[WRONG
White is caught in a ladder, but it works for him.
])))
(;B[of]CR[of]
;W[pg]CR[pg]
;B[ph]CR[ph]
;W[og]CR[og]C[CHOICE WRONG
White is caught in a ladder, but it works for him.])
(;B[pg]CR[pg]
;W[of]CR[of]
;B[nf]CR[nf]
;W[og]CR[og]C[CHOICE WRONG
White is caught in a ladder, but it works for him.]))
(;B[of]CR[of]
;W[oe]CR[oe]
;B[ne]CR[ne]
;W[nd]CR[nd]
;B[md]CR[md]
;W[od]CR[od]
;B[oc]CR[oc]C[CHOICE RIGHT
Stones are caught -thanks to Andre Engels for this one
- use navigate for variations.]))